In an automatic transmission for an automobile, the power of an engine is transmitted to the input shaft of a transmission mechanism via a torque converter and the rotation of this input shaft is varied and is transmitted to an output shaft thereof by this transmission mechanism to rotate and drive a driving wheel. In the commonest transmission mechanism, a plurality of gears are arranged between the input shaft and the output shaft to construct a plurality of power transmission paths of different transmission gear ratios between the input shaft and the output shaft and friction engaging components of clutches and brakes are mounted in the respective power transmission paths. By individually controlling hydraulic pressures applied to the respective friction engaging components according to a request to switch a shift step, the filling control of filling specified friction engaging components with hydraulic oil and the draining control of draining the hydraulic oil from specified friction engaging components is performed to selectively switch between engagement and disengagement of the respective friction engaging components to switch the power transmission paths between the input shaft and the output shaft to thereby switch a transmission gear ratio.
In this automatic transmission, when a request to switch to a new shift step is made during the operation of switching a shift step and a target shift step is changed, the multiple shift control of switching a shift step of the transmission mechanism to the target shift step after shift change is performed. For example, the following case is assumed; the filling control of filling the cylinder of a clutch or the like with hydraulic oil is being performed and the filling control is switched in the process of the filling control to the draining control of draining the hydraulic oil by the change of a target shift step and then the draining control is again switched in the process of the draining control to the filling control by the change of a target shift step. In this case, depending on a state where the hydraulic oil is drained from the cylinder by the draining control before shift change, there is a possibility that when the cylinder is again filled with the hydraulic oil, the cylinder is excessively filled with the hydraulic oil to suddenly increase a force for engaging the clutch to cause a shift shock.
As a technology for preventing the shift shock caused by this multiple shift, for example, as disclosed in Japanese Patent No. 32919790, there is proposed a technology that when a request to switch to a second shift step is made during the operation of switching to a first shift step, the operation of switching to the second shift step is delayed until the operation of switching to the first shift step is finished.
However, the operation of switching to the second shift step is delayed until the operation of switching to the first shift step is finished and hence it takes a long time before the operation of switching to the second shift step is finished.